The present invention relates to printing inks and more particularly to gravure printing inks. Hence forth in this text, the item "printing ink" means printable materials with actual color-rendering power as well as non coloring lacquers and non coloring printing ink-varnishes.
In prior art rotary printing processes, especially gravure printing processes, a solvent-containing ink is applied to a printing cylinder which is preformed with the desired image to be printed. By impression, which is exerted by an impression-roller, the ink then is transferred to the web, which is running in the narrow gap between the impression roller and the printing cylinder. Finally, the imprinted web is fed into a drying section of the printing machine, where the solvent is removed from the web as well as from the ink-layer, which lies straight on the web or partially penetrates it. The spatial dimensions of those construction elements of the overall printing machine, which are necessary for the particular task of printing itself, are small compared to the spatial demands of the drying section and the construction elements, which are necessary for realization of the drying technology. This holds true especially for the gravure printing process and also for the web-fed offset printing technique.
Hence, a first disadvantage of the prior art printing processes and the printing machines, which achieve drying of the print by forced evaporation of the solvent content of the ink, especially concerning gravure and web-fed offset printing, has to be traced back to the relatively large space requirement of the drying section and the related peripheral construction elements, which are indispensable for drying success. With enhanced printing speeds, the shortening of the drying time per unit length of the running web has to be counteracted by an increasing length of the drying duct and hence by even larger spatial demands of the related machinery.
In the case of rotary gravure printing, the drying section generally includes a manifold of pipes, which are arranged in parallel to the running web. These pipes are equipped with nozzle-arrays, by means of which impinging jets of heated air are blown onto the running web. The high efficiency of heat- and mass-transfer, which has to be established across the boundary, may only be achieved by a highly turbulent field of flow from the impinging jets. This turbulence in turn results from the high blast delivery of the fans, whose intake comes from the surroundings and is heated either actively by means of heat exchange to up to 80.degree. C. if needed, or passively by flow resistance in the ducts to nearly 40.degree. C. or so. With increasing printing speed or machine productivity, the energy-input needed for the drying process must also be increased. This holds true with respect to the mass flow and hence the mechanical power of the fans as well as for the thermal power of heat generation, be it active or passive. In fact, the drying air represents the biggest mass-flow of all materials fed into or leaving a fast running gravure printing machine.
The heat-and mass-transfer, which to a large extent is influencing the production costs is neither physically bound nor constitutional to the print-product like paper or the ink and hence in principle could be dispensable. The high blown and expensive drying periphery of a printing unit, which has been described so far mainly in terms of rotogravure printing, is as essential to a large extend for web-fed offset as well as rotary screen-printing. In web-fed offset printing, the thermal power needed for the drying process is especially high.
Hence, another disadvantage present with printing processes operating with physical evaporation drying is a high expense of power for the forced convection and the thermal input.
For mainly two reasons, only a limited partition of the vapor-carrying drying air may be fed back into the drying duct. The first reason is, that in the course of extensive or nearly complete feedback, the vapor concentration of the then largely unrefreshed drying air would come to saturation and thus the dew-point and hence zero-drying efficiency would be reached. The second reason is, that in the case of ignitable vapors, the lower limit of explosion (LEL) quickly would be reached or even be exceeded. Instead of, an abundant safety-margin should strictly be observed, especially in gravure printing. For economical reasons as well as for environmental considerations, the vapor-carrying air may not be exhausted. Instead of, it is fed into a solvent-recovery plant or subjected to thermal combustion.
The construction components for solvent recovery or combustion impose high demands for space and reach very bulky dimensions in the case of large printing plants. Obviously, the operation of these facilities is also very cost-demanding. The same holds true for thermal combustion.
Thus, a third serious shortcoming of printing processes using solvents and evaporation-drying is the effort and the costs, which have to be spent for solvent recovery or thermal combustion.
In order to increase the efficiency and the economics of the solvent recovery and also in order to ensure the vapor-concentration in the working area falling short of the maximum tolerable working-place concentration, the printing machines as well as the drying equipment and all the related periphery are fully encapsuled. An additional requirement, which has to be met due to the flammability of most solvents fed into printing business, is, that all electrical equipment, which is exposed to solvent vapors inside the printing units or outside in the drying ducts, needs to fulfill the explosion- hazard specifications, which are imposed by law.